Spong, U.S. Pat. No. 4,097,895 issued June 27, 1978, has described an ablative recording medium which comprises a light reflecting material, such as aluminum or gold, coated with a light absorbing layer, such as fluorescein. A focussed intensity modulated laser beam, as from an argon or helium-cadmium laser, when directed at the recording medium, vaporizes or ablates the light absorptive material, leaving a hole and exposing the light reflecting layer. The thickness of the light absorptive layer is chosen so that the structure has minimum reflectivity. After recording then, there will be maximum contrast between the minimum reflectance of the light absorbing layer and the reflectance of the light reflecting layer. Further, when the light reflective material is itself a thin layer on a non-conductive substrate, since little energy is lost through reflection from the thin absorbing layer, and little energy is lost by transmission through the reflecting layer, the energy absorbed from the light beam is concentrated into a very thin film and recording sensitivity is surprisingly high.
This system operates very well, but has the disadvantage that the argon and helium-cadmium lasers are bulky devices that require a comparatively large amount of electrical input power to operate them. In addition, an external light modulator is required. It would be desirable to operate at lower electrical input power levels such as those required by solid state injection lasers, including the aluminum gallium arsenide laser. These lasers operate between about 750 and 850 nanometers (nm) and thus materials which absorb at these wavelengths would be required for recording media useful in the above recording system.
In order to be useful as a light absorbing layer, materials must be able to be applied to form a thin, smooth layer of optical quality and a predetermined thickness; they must be absorptive at the frequency of the optical source employed; and they must ablate or melt to form smooth holes to give a signal pattern having a signal to noise ratio of at least about 40 decibels (dB).